1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus. More particularly, the invention relates to a zoom lens and an image pickup apparatus each having a small size, high image quality, and high zoom ratio.
2. Description of Related Art
Recently, small-sized image pickup apparatuses such as video cameras and digital video cameras for consumer use have also been widely available for home use.
For these small-sized image pickup apparatuses, the demand for small size enhancing portability, high image quality, high zoom ratio, and the like is increased. Similarly, a photo-taking lens mounted on an image pickup apparatus, particularly a zoom lens, are required to satisfy the miniaturization by reducing the entire length and depth, and the improvement of lens performance.
There have also recently been strong demands in optical camera-shake correction. The design difficulty to satisfy the requirement of the optical camera-shake correction in addition to the miniaturization, high image quality and high zoom ratio has also become increasingly higher.
Under these circumstances, some of known zoom lenses are configured to arrange, in an order from the object side to the image side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a negative refractive power, and a fifth lens group having a positive refractive power. The third lens group includes a lens having a positive refractive power and a lens having a negative refractive power. In order to correct image variations during camera-shake, the third lens group is movable in a direction substantially orthogonal to an optical axis direction (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-228001(Patent Document 1)).
The zoom lens described in the Patent Document 1 provides, for example, a zoom lens for a video camera capable of performing optical camera-shake correction by the configuration described above.
On the other hand, some of known zoom lenses are configured to arrange, in order from the object side to the image side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a positive refractive power, and a fifth lens group having a positive refractive power. The fifth lens group includes a positive sub lens group having a positive refractive power and a negative sub lens group having a negative refractive power. In these zoom lenses, the image is shifted to permit camera-shake correction by employing the positive sub lens group which is movable in a direction substantially orthogonal to an optical axis (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-23593(Patent Document 2)).
In the known zoom lenses as described in the Patent Documents 1 and 2, it is designed particularly on the telephoto side so that a part of the lens groups constituting the zoom lens, for mainly correcting the image blur due to camera-shake, is movable in the direction substantially orthogonal to the optical axis so as to achieve high quality image and optical performance improvement. Further, the lens configuration is determined to meet the desired optical performance while ensuring small size and high zoom ratio.
Thus, in the known zoom lenses as described in the Patent Documents 1 and 2, owing to the lens configuration consisting of the five lens groups, the optical camera-shake correcting function can be ensured to meet the desired superior optical performance, while achieving high zoom ratio and high image quality.
However, in the known zoom lenses described above, particularly the mounting of the optical camera-shake correcting function increases the size of the camera-shake correcting mechanism. This cannot be disregarded for reducing the size of the image pickup apparatus including the mechanism thereof, thereby causing the following issues.
That is, in the zoom lens described in the Patent Document 1, the third lens group can be movable in the direction substantially orthogonal to the optical axis direction in order to correct the image variations during camera-shake. The third lens group has a tendency that a light flux diameter becomes larger than the lens groups other than the first lens group. This extremely increases the effective diameter on the lens surface covering the camera-shake correction, which leads to a large apparatus.
The third lens group exists at substantially the center of the optical axis in the zoom lens and has strong refractive power. Therefore, when the third lens group is moved in the direction orthogonal to the optical axis direction, these other lens groups produce wide fluctuations of light flux position, thereby increasing the effective diameter on the lens surfaces constituting these other lens groups, which results in a large image pickup apparatus.
The zoom lens described in the Patent Document 2 can perform camera-shake correction by shifting the image under the configurations that the fifth lens group is composed of the positive sub lens group having the positive refractive power and the negative sub lens group having the negative refractive power, and that the positive sub lens group is movable in the direction substantially orthogonal to the optical axis direction. However, the application of the camera-shake correcting mechanism to the object-side lens group (the positive sub lens group) of the fifth lens group necessitates space for arranging the camera-shake correcting mechanism on both sides in the optical axis direction of the positive sub lens group, and the size of the image pickup apparatus is correspondingly increased.
The optical design is limited to ensure the space on both sides in the optical axis direction in the positive sub lens group. This might cause image quality degradation.
Further in the zoom lens described in the Patent Document 2, the cemented lens system is formed in the image-side lens configuration of the second lens group and the succeeding ones. Particularly in terms of optical performances such as chromatic aberration and resolution on the wide-angle end, various aberrations are not sufficiently corrected from the viewpoint of the optical performances to ensure superior high vision image quality. Therefore, various aberrations such as on-axis color aberration and magnification color aberration remain, thereby exerting adverse effect on image quality.
The above-mentioned issues are aggravated by ensuring high zoom ratio and high image quality, and remain unresolved from the viewpoint of realizing a small-sized and high zoom ratio zoom lens.
Accordingly, it is desirable to provide a zoom lens and an image pickup apparatus capable of achieving miniaturization, high image quality, and high zoom ratio.